The Real (and Virtual) Adventures of Nathan the IT Guy

To think that technology has come so far in the last 10 years, and now RAID arrays are becoming a risk! This is information everyone should read…

The problem with RAID 5 is that disk drives have read errors. SATA drives are commonly specified with an unrecoverable read error rate (URE) of 10^14. Which means that once every 200,000,000 sectors, the disk will not be able to read a sector.

2 hundred million sectors is about 12 terabytes. When a drive fails in a 7 drive, 2 TB SATA disk RAID 5, you’ll have 6 remaining 2 TB drives. As the RAID controller is reconstructing the data it is very likely it will see an URE. At that point the RAID reconstruction stops.

Here’s the math:
1 – 1 /(2.4 x 10^10)) ^ (2.3 x 10^10) = 0.3835

You have a 62% chance of data loss due to an uncorrectable read error on a 7 drive RAID with one failed disk, assuming a 10^14 read error rate and ~23 billion sectors in 12 TB. Feeling lucky?

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About This Blog

Nathan is a Senior Technical Architect and he’ll be blogging about virtualization, portable apps, useful little-known apps and general IT issues and resolutions encountered in his role as an IT consultant. He’ll also be blogging about his certification exam experiences with the MCTS track. Currently he has VCP3,4, 5, VTSP4/5, VMware VDI Accredidation, and MCP Certifications.